An American freelance journalist has been killed in the southern Iraqi city of Basra, four days after he wrote an opinion piece for a U.S. newspaper criticizing the rise of Shi'ite Muslim fundamentalism in the south. Meanwhile, the restive western Anbar province, 14 U.S. Marines were killed Wednesday, in one of the single deadliest attacks against U.S. forces since the beginning of the war.

Police in Basra say the body of freelance journalist Steven Vincent was found dumped on the side on the highway south of Basra late Tuesday, just hours after Mr. Vincent and his female interpreter were seized at gunpoint.

Mr. Vincent had been shot multiple times. His interpreter was also shot several times, but survived. She has been hospitalized in serious condition.

U.S. Embassy officials in Baghdad say they are working with the British military and local Iraqis in Basra to determine who carried out the shootings.

The journalist, whose work has appeared in major American newspapers and magazines, had been in southern Iraq for several months, researching a book. His last article was an opinion piece, published in the New York Times newspaper four days ago.

In the article, Mr. Vincent described Basra as a city that was increasingly coming under the control of Shi'ite Muslim religious groups. He quoted an Iraqi police official who said that the city's police force had radical Shi'ite militia members among its ranks, who have been carrying out numerous assassinations of former Baath Party members in Basra.

During Saddam Hussein's regime, the country's majority Shi'ites were oppressed and persecuted by the ruling, Sunni Muslim-dominated Baath Party.

A leading secular Shi'ite politician, Mithal al-Alousi, says he can confirm that Shi'ite militia groups are active in Basra and believes they had a strong motive for killing Mr. Vincent. Still, the politician says it is difficult to know who actually pulled the trigger.

"I'm not sure what has happened," he said. "But I'm very sure we do have Iranian influence in Basra. We have to study the case and ask, 'Who really has the security control of this area?'"

In the west of Iraq, a large roadside bomb exploded Wednesday near a U.S. Marine amphibious assault vehicle as it traveled south of the town of Haditha.

The blast killed 14 Marines and a civilian interpreter. It was the deadliest attack on U.S. troops since last December when a suicide bomber killed 22 people, including 14 U.S. servicemen, inside a mess hall in the northern city of Mosul.

The town of Haditha, roughly 200 kilometers northwest of Baghdad in Anbar province, was also the site of a fierce battle two days ago in which six Marines were killed in clashes with militants.

A seventh Marine was killed by a car bomb blast in the town of Hit, southeast of Haditha.

An Iraqi extremist group with ties to al-Qaida, Ansar al-Sunna, claimed responsibility for Monday's attacks against the Marines. Anbar province includes the towns of Fallujah and Ramadi and has long been the center of Iraq's Sunni-led insurgency.